In several two-way communication systems, subscriber units are required to share limited communication resources (i.e., communication channels or time slots). To control the allocation of the limited communication resources, a resource controller receives a call request from a subscriber unit to communicate with one or more other subscriber units. A communication (call) may not commence until the resource controller transmits a communication grant code that informs the communication participants as to the appropriate communication resource designated for their use.
When a communication involves the transmission of sensitive or confidential information, subscribers generally prefer to encode or encrypt their transmissions to prevent unauthorized parties from monitoring the communications. To do this, subscribers must enter an encrypted operational mode that engages an encryption device or circuitry in the subscriber unit to encrypt (and decrypt) the transmissions. However, this added information security feature may actually frustrate and imped effective communication since the several subscriber units may select their operational mode independently of one another. That is, a subscriber initiating a communication will not know whether the called subscriber units are operating in a compatible operational mode so as to be able to participate in the communication.
Another problem with limited resource communication systems arises when a requesting subscriber unit may have to wait to access to a communication resource. During this time, the subscriber unit may change its operational mode, or be called to participate in another communication so as to be unable to participate in the communication originally requested. Finally, when there are multiple operational modes on a communication system, there is a risk that a subscriber will change operational modes in mid-communication, which may compromise confidential or proprietary information due to an erroneous transmission. Accordingly, a need exists for a method for preventing undesired or erroneous transmission, while providing the added security of encrypted (or encoded) transmissions.